Merge branch 'fix/opm-for-3.2' into 'releases/3.2'

Fix OPM for 3.2

See merge request !2069

cmake.opts

@@ -36,6 +36,7 @@ OPM_FLAGS=""
 # to build opm it might be necessary to set manually the following variables (comment the above line)
 #OPM_FLAGS="
 #-DUSE_MPI=ON
+#-DOPM_CLANG_WITH_STDC++FS=ON
 #"
 
 # set this to "ON" if you want to be able to have the headercheck target

cmake/modules/DumuxMacros.cmake

@@ -14,3 +14,7 @@ find_package(PTScotch)
 find_package(PVPython)
 find_package(Valgrind)
 find_package(Quadmath)
+# The following is required for being able to depend on OPM.
+# Remove once a better solution has been found at
+# https://github.com/OPM/opm-common/issues/1751.
+find_package(OpenMP)